8 ms·

The prediction is really that these stars collided about 1795 years ago, and we may find out about it soon.

• drjesusphd 10y ago
  Because simultaneity itself is relative, in our frame it actually does happen in 2022. It doesn't make sense to say the stars already collided if it's not in our past light cone.
  • arithma 10y ago
    But we do say we see the Sun as it were 8 minutes ago because of distance/c. Are you suggesting that this notion be removed as well, because I agree with you regarding the light cone, but the Sun being 8 minutes away makes too much sense for me to abolish the concept. Is there any philosophical speak about this topic or is it basically resolved?
    • deleted 10y ago
      [deleted]
    • random314 10y ago
      There exists a reference frame in which the distance between the Sun and earth is so compressed by Lorentz contraction that the light leaves the Sun and arrives at the earth in 1 second. In relativistic terms, the photon leaving the Sun and arriving at the earth can be considered to be simultaneous.
      • eternalban 10y ago
        > There exists a reference frame But is that the reference frame that we experience?
        • random314 10y ago
          Obviously, no.
      • euyyn 10y ago
        > In relativistic terms, the photon leaving the Sun and arriving at the earth can be considered to be simultaneous. No they cannot, unless you boost to a reference frame that's moving at the speed of light, which is a useless frame. In all other frames, including the ones we are in, the photon left the Sun before arriving to Earth.
      • madgar 10y ago
        > In relativistic terms, the photon leaving the Sun and arriving at the earth can be considered to be simultaneous. You just constructed an argument that can be readily used to argue that all events are simultaneous. That should flip your internal bozo bit and help you see that the argument is not sound.
        • eternalban 10y ago
          If you feel like getting mystical about it, for a hypothetical Photonic Being all events are in fact simultaneous: It is always 'now' and per various mystic schools, that 'timelessness' is the actual meaning of 'Eternal'.
        • random314 10y ago
          > You just constructed an argument that can be readily used to argue that all events are simultaneous No, I haven't. I have studied relativity in University, have you? Lol @bozobit I will make the statement more rigorous. "Let event A represent the emission of a photon. Let event B represent the arrival of a photon elsewhere. Event here refers to a unique location in space-time i.e (x,y,z,t). As per special relativity, you can always find a reference frame such that the time separation between A and B can be arbitrarily small. It can't be made exactly zero, but it can be made as close to zero as you wish to." Use this formula to figure out the reference frame of interest. https://en.wikipedia.org/wiki/Length_contraction https://en.wikipedia.org/wiki/Length_contraction The speed of light is constant. So decrease/contract the length until the time required = distance/c is one second or less.
  • bottled_poe 10y ago
    I'm not a physicist, but pretty sure you are conflating two different physical phenomena... One is the delayed observation due to the limited speed of light, the other is reference frame distortions due to special relativity.
    • random314 10y ago
      I'm a physicist. OP is correct. OP is not conflating 2 different phenomena. The light cone is defined by the speed of light and hence they are one and the same phenomenon.
      • ewjordan 10y ago
        You're right that they're not conflating the phenomena, but to say that two events are only simultaneous if they're on the light cone is still incorrect - that's the boundary of the region of simultaneity, whereas the OP was suggesting that it's the entirety of it. Since the light hasn't reached us yet, it's perfectly valid to state that in some frame of reference, the event is happening right now. 2022 is the last possible time at which we could make that claim because the events become causally separated, which is I think what OP was getting at, though the wording was a bit off.
      • zorpner 10y ago
        OP is not correct so... I guess you're not a physicist? Logic is rough, man.
      • pif 10y ago
        I'm a physicist, too. And I don't understand what you and OP are talking about. In out frame, the collision happened ~2000 years ago. The light will arrive to Earth in a couple more years. That's it. By the way, what's the need of talking about frames? Which other frame do you have in mind apart from ours?
        • TeMPOraL 10y ago
          IANAP, but doesn't speed of light actually define "at the same time"? I.e. for all intents and purposes, it's happening "now" for us, because there's no meaningful way of getting there faster than light. In a way what I'm imagining is speed of light being a kind of "clock signal" of digital electronics, except continuous, not discrete.
  • markfluffalo 10y ago
    Don't you need to be comparing two events? What are these?
  • solipsism 10y ago
    No, you are confused. The big bang is not happening right now "in our frame" since we can turn on our radios and hear the CMB.
    • random314 10y ago
      OP is not confused. He is not talking about the Big Bang which also involves expansion of space.
  • imh 10y ago
    But in a classic Minkowski diagram like this one (https://en.wikipedia.org/wiki/File:Minkowski_lightcone_lorentztransform.svg https://en.wikipedia.org/wiki/File:Minkowski_lightcone_loren...), don't you define the horizontal line (or at least a single line) as simultaneous in a given frame, instead of the entire white region? In all frames, the blue region are past or future, but in our particular frame, we can say more than that, like when the event happened. As far as I can remember, the star blowing up and the light reaching us are only simultaneous in the light's frame.
    • contravariant 10y ago
      That is a diagram of empty space, figuring out what that horizontal line represents in places with strong gravity is a bit tricky. In fact I'm not 100% sure there's a way to define it uniquely. You could define the global time for a position to be the time at which light it sends reaches the earth, minus the time it took for the light to travel, but with things like gravitational lensing this can have some weird results, and may in fact not be well defined.
      • imh 10y ago
        I get that it might be hard/impossible to uniquely define a single correct 3D subspace of the full 4D spacetime we're in as simultaneous to a given point in a given frame, but surely simultaneity is a 3D notion, right? This 3D spacelike subset of spacetime is simultaneous in some frame (or set of frames)? I imagine that if you define everything on the boundary of your past light cone as simultaneous to you, it requires that everything on your future light cone to be simultaneous too, which would mean that everything between them is also simultaneous. That kind of definition defines a 4D subset of spacetime as simultaneous. In that set are points in each others future and past light cones, which we can unambiguously say are not simultaneous to each other. I think that's the heart of the argument everyone's having. I never took GR, so maybe you can shed some light on it (heh) from that perspective?
  • deleted 10y ago
    [deleted]
  • marcoperaza 10y ago
    That's just nonsense. We know the distance between us and the star, we know that light travels at c, and therefore we know that, in our reference frame, the event being observed happened 1795 years ago.
  • leni536 10y ago
    > simultaneity itself is relative That's right. > in our frame it actually does happen in 2022 That's not. Read on clock synchronisation: https://en.wikipedia.org/wiki/Einstein_synchronisation https://en.wikipedia.org/wiki/Einstein_synchronisation
  • mirimir 10y ago
    Well, maybe so, theoretically. But in fact, there may be multiple light paths between us and remote stuff. And the length of those light paths is in constant flux. Indeed, having data from multiple light paths has proved very useful. So it seems sensible to talk about stuff happening in its own frame, and distinctly about observation delays.
  • danbruc 10y ago
    No, it happened 1795 years ago. There is only one frame of reference involved here, one we can attach to the Milky Way neglecting the relatively small movements of earth and KIC 9832227 within it. And two events are simultaneous in a given frame of reference if light emitted from both events reaches a point midway between the locations of the two events at the same time. So you position your space ship in the middle between were earth and KIC 9832227 were 1795 years ago, about 900 light years away from each and sometime about the year 1122 you will see the stars colliding and at the same time, with a good enough telescope, what was happing on earth in the year 222 which establishes that the the collision of KIC 9832227 and the year 222 on earth were simultaneous in the frame of reference attached to the Milky Way.
    • dsqrt 10y ago
      Please see my other reply below. Reference frames are local constructs. There is no such a thing as a (unique) reference frame of the entire galaxy that can be used to define simultaneity. In fact there are infinitely many of them.
      • danbruc 10y ago
        I have seen that your profiles says theoretical astrophysicist so you certainly know a lot more than me about the topic. Nonetheless I can not see why we can not have a reference frame extending over 100,000 light years or at least over 1,800 light years. That seem relatively small distance, the relative velocities are relatively small, space is relatively flat, expansion of space is, I guess, not really relevant within galaxies. So naively I would expect that you can attach a reference frame to almost all objects in the Milky Way and they would agree on simultaneity to within hours or maybe days. There are certainly some notable exceptions like the central black hole or particles traveling at an appreciable fraction of the speed of light and you can certainly just invent a reference frame with huge relative velocity changing simultaneity a lot. But are there really places within the Milky Way so that simultaneity would be off by hundreds and thousands of years over a distance of just 1,800 light years? If yes, what is the cause of that, as far as I can tell it would have to be an effect of general relativity because, again as far as I can tell, there is no problem of defining simultaneity across extended distances in special relativity and the involved velocities are not large enough to have an appreciable effect to begin with. EDIT: Just calculated an example Lorentz transformation, 500 km/s relative velocity and 1,800 light years distance, and the time difference comes out at almost exactly 3 years. That is certainly more than I expected and adds up to 166 years across the entire Milky Way, on the other hand 500 km/s is probably quite a bit above common relative velocities within the Milky Way. So I am still not convinced that picking a reference frame for the entire Milky Way, say with the origin at the center of mass, one axis coinciding with the axis of rotation and rotating with the Milky Way so that the angular momentum vanishes, could not provide a good enough reference frame for the entire Milky Way for back of the napkin calculations.
  • will_pseudonym 10y ago
    "If two stars collide in the universe with no one around to see them..."
  • pif 10y ago
    Simultaneity is only defined when you talk about two (or more) events. The discussion was about an event: the collision of two stars. What is the other event?
    • jstanley 10y ago
      The observation of the collision?
    • jacobolus 10y ago
      The generous interpretation of what he means is that considering that this event is outside of our past light cone, there exists an inertial reference frame such that the collision happens “after” our “present”. Of course, relative to our own inertial reference frames as we experience them, i.e. where we are mostly not moving, the collision would be thousands of years in the “past”. When he says “in our frame” he is clearly at least slightly confused.
      • lawless123 10y ago
        It's like saying lighting struck the moment you heard the sound.
  • tcgv 10y ago
    No it doesn't happen in 2022, otherwise it would take another 1795 years for the light of that event to reach us. If we see the collision in 2022 it means it happened in the distant past.
• tfgg 10y ago
  That's true in our reference frame, with some error bar, but astronomers rarely make the distinction when talking about observing events, so I always find it odd when laymen insist on making it. I guess the distinction is rarely made by professionals because - It requires defining a reference frame. - It makes no difference to the causality. - Time is a function of distance, which usually has large error bars on it, so it's better to discuss the distance instead. - Time as a function of distance gets more complex on cosmological scales. On the last point, you can get the exciting realisation that, due to the expansion of space, objects actually start getting larger the further away they get beyond a certain distance.